Alastair Campbell's diary is a 750-page heavyweight that can be boiled down to a single sentence: "How me and Tony stuffed the media and changed the world".

Filleted of sharp substance, the book omits not just the reality of Tony Blair's dealings with Gordon Brown - understandable with Brown newly in power - but also the true nature of Campbell's dealings with the press. This is much less forgivable. The book contains astonishingly few references to journalists with whom he was in daily contact.

Reading the diaries is like peering through a partly-curtained window. Occasionally people pass into view, or raise their voice so they can be heard through the glass. But a lot is missing.

Only rarely does he record the real art of managing the media: "The words went out to Webster, the spin was applied and away we went".

To Campbell's credit he makes no attempt to pretend otherwise. Put aside the hunt for stories and a more interesting book emerges. It captures strikingly the laddish, hungry, boastful side of New Labour, a thuggish competition to acquire and use power. The details are realistic and for the most part depressing.

Women suffer in particular. Clare Short - "God, she does turn my stomach" - and Cherie's strange companion Carole Caplin (Campbell spotted her flaws early). He is nasty about Cherie, too. There is a general sense of picking on victims: Campbell is far crueler about those who cannot hit back than those who can. He loves power and the men who hold it.

This was not a world for wimps: everyone, especially Campbell, swears. He tells Blair that the section of the diaries sent to the Hutton inquiry contains "a fair bit of bad language". "Fuck?" asks Blair. "Yes." "Cunt?" "Probably." "Bloody hell, Alastair."

He revels in brutality: he wants to "kill Gilligan" and suggests in a note passed across the cabinet table that Blair shoots Short in the same way "Saddam shot his health minister". He offers to "get a gun". Blair's joking reply: "Yes".

And he hates the media. His contempt for political journalism is absolute: even in 1997 he records "I was sick of dealing with wankers".

This obsession blinds him at times - he regards news that the dodgy dossier on Iraqi weapons was copied off the internet as just "another spin story", missing completely the harm it did to his boss and trust in politics. Campbell is at his lamest here, merely sending a note "to make clear the importance of quality control". Meanwhile, Britain went to war.

It is hard not to find something odd about a man who tries to block freedom of information, but then betrays confidences by publishing his diaries. Or someone who complains repeatedly of "a political press ... obsessed with the trivia", but fed it constantly. As a Mirror hack, it was Campbell, after all, who spotted the comic potential of John Major's underpants (in the diaries Blair's own yellow and green underpants appear).

But it is difficult to read Campbell's book without agreeing, reluctantly, with many of his judgments about the impact of the media - "the babble industry", he calls it, a nice phrase - on the British political process. It is often degrading. What Campbell fails to spot is that he has always been part of the degradation.

He often writes as though he were Blair's chief of staff, not his press officer. But his role was to create a presentable impression of the truth. Tellingly, Stephen Byers' adviser, Jo Moore, is criticised not for thinking that 9/11 was a moment to bury bad news, but for saying it.

Above all, Campbell will be remembered for presenting the Iraq war in a manner that was palatable to the British public. This seems to have warped even the prime minister's understanding of the reality of conflict. "God, it is awful this war business," Blair says in an aside when eight marines are killed, as if the thought were new. Only when David Kelly dies, and Campbell's world collapses, does he seem to understand what he had created.

The names of many great political diarists begin with a C: Colville, Channon, Crossman, Castle and Clark. Can Campbell's name be added to the list? It is impossible to think of anyone considering the Blair years from now on without it. If his aim is to win sympathy, he fails. But perhaps that is a sign of the book's underlying accuracy.

Nasty, brutish and long, Campbell's diary is the edited outpouring of an obsessive, but its significance cannot be denied.